1. Field of the Invention
The present invention relates to a power transmitting system for motor vehicles and more particularly to a four wheel drive system for motor vehicles. More specifically, the present invention pertains to a torque splitting mechanism for distributing the driving torque to the front and rear wheels with a predetermined torque splitting ratio.
2. Description of the Prior Art
It has been known in the art of four wheel drive vehicles to provide a torque splitting mechanism for distributing the engine drive torque between the front and rear wheels with a predetermined torque split ratio. For example, in Japanese patent application No. 56-155853 filed on Sept. 29, 1981 and disclosed for public inspection on Apr. 4, 1983 under the disclosure No. 58-56920, there is disclosed a vehicle drive system provided with an intermediate transfer clutch for adapting the vehicle alternately to a two wheel drive type or to a four wheel drive type. The engagement of the transfer clutch is controlled so that the torque transmitting capacity of clutch is changed to thereby change the torque split ratio between the front and rear wheels.
It should however be noted that in the mechanism as disclosed by the Japanese patent application there is a problem of the clutch facing material being rapidly worn since the transfer clutch is frequently operated with a partly engaged condition. Further, the frictional force produced in the transfer clutch changes depending on the ambient temperature and the humidity. Besides, the condition of engagement of the transfer clutch is influenced by the vibrations of the vehicle body. Thus, the control of the torque split ratio in the proposed system is not accurate and reliable to a satisfactory extent.
In the Japanese patent application No. 59-182577 filed on Sept. 3, 1984 and disclosed on May 31, 1986 under the disclosure No. 61-62641, there is disclosed a motor vehicle four wheel drive mechanism which includes a plurality of differential gear sets provided on the front axle and a selecting device for selecting a desired one of the differential gear sets to establish a desired torque split ratio between the front and rear wheels. In the European patent application No. 85306877.3 filed on Sept. 27, 1985 and published on Apr. 9, 1986 under the publication No. 0177286, there is disclosed a similar vehicle driving system. The European patent application corresponds to the Japanese patent application Nos. 59-202442 filed on Sept. 27, 1984 and 59-201815 filed on Dec. 24, 1984.
The driving systems as proposed by these applications are considered as being free from the problems inherent to the driving system having the aforementioned intermediate transfer clutch. These systems are not however satisfactory because a complicated mechanism is required for providing two or more differential gear sets on a front or rear drive axle. It should further be noted that a plurality of differential gear sets provided on a single drive axle will make it necessary to increase the length of the drive axle and to thereby decrease to corresponding extent the lengths of the wheel drive shafts which are connected to the drive axle. Such decrease in the lengths of the drive shafts will cause an undesirable increase in the angle between the drive axle and the drive shafts.